TLR4, or Toll-like receptor 4, is a protein that plays a crucial role in the immune system. It is part of a family of receptors that help the body recognize pathogens, such as bacteria and viruses. When TLR4 detects specific molecules from these pathogens, it activates immune responses to help fight off infections. TLR4 is primarily found on the surface of immune cells, such as macrophages and dendritic cells. By recognizing components like lipopolysaccharides from bacterial cell walls, TLR4 triggers signaling pathways that lead to inflammation and the production of cytokines, which are essential for coordinating the immune response.